Methods of preparing crosslinked polymers, and anion and cation exchange resins derived therefrom, which are characterized by macroporous structures and which show improved properties compared to traditional gel resins, are known. In contrast to conventional gel resins formed by copolymerization of monovinylidene and polyvinylidene monomers and characterized by the presence of micropores, the pore structure being the distance between the crosslinked polymeric chains, macroporous resins contain a significant nongel porosity in addition to normal gel porosity. This nongel porosity arises from channels present between the gel lattice. These microscopic channels are separate and distinct from the micropores, which are present in all crosslinked copolymers, as is well known to those skilled in the art. While the channels are themselves relatively small, they are large when compared with the micropores of the prior art, gel type resins.
A number of patents have issued on macroporous resins and various methods for generating macroporosity. The terms macroporous, macroreticular, sponge-like and channelled have been used, more or less interchangeably, by those skilled in the art to characterize the opaque beads and resins. Pore-forming, phase-separating, precipitant and porogen have, likewise, been used to refer to the agent used to produce the macroporous structure.
British Patent 785,157 and U.S. Pat. No. 3,122,514 disclose the use of pre-polymerized styrene dissolved in mixtures of divinylbenzene, styrene and a polymerization initiator yielding opaque beads with microchannels and pores through which molecules can diffuse more readily than in conventional resins. Ion exchange resins prepared from these copolymer beads are disclosed in British Patent 785,157 as being much more resistant to shock, superior as anion exchangers in color removal and, as cation exchangers, were directly hydratable after sulfonation with little or no breakage. However, this technology did not improve resistance to oxidative de-crosslinking of the polymeric matrix itself and introduced the problem of leaching of the original linear polystyrene additive in cation exchange products produced by this process. That is, the linear polymer chains, which are not crosslinked, become on sulfonation at least partially water soluble and leach from the resin, causing contamination of any fluid passing through the resin. Such resins not only contaminate the fluids they were intended to purify but also process fluids, which affects production economics adversely since reagents cannot be recycled. Moreover, environmental pollution must be addressed. In the process of U.S. Pat. No. 3,122,514 linear polystyrene is either formed-in or dissolved-in the styrene monomer mix which is thereafter polymerized. The linear polystyrene that is tangled into the crosslinked copolymer bead is then leached out of the bead leaving channels through the bead.
U.S. Pat. Nos. 3,367,889 and 3,627,708 disclose a system whereby an emulsion of water in styrene-divinylbenzene is polymerized as droplets in a surrounding aqueous suspension. These patents disclose the use of water in combination with a surfactant as the porogen in forming macroporous resins.
U.S. Pat. Nos. 4,221,871, 4,224,415, 4,256,840 and 4,501,826 are each directed to methods of preparing crosslinked macroreticular resins, containing at least one monovinylidene and a polyvinylidene monomer, which are phase separated by the use of a precipitant in the monomer mixture during aqueous suspension polymerization. The precipitant is disclosed as being either a liquid or solid in its natural state, the preferred class of precipitants being liquid under the polymerization conditions. Suitable precipitants are materials which are solvents for the monomers being copolymerized but exert essentially no solvent action on the copolymer. Examples include butanol, sec-butanol, tert-amyl alcohol, 2-ethylhexanol and decanol. By adding the precipitant to the monomer phase, the solubility of the. copolymer formed is decreased and the copolymer separates from the monomer phase as it is formed. This phenomenon is referred to as `phase separation`. As the concentration of monomer in the polymerizing mass decreases due to polymerization and as the concentration of the precipitant increases thereby, the newly formed polymer is more strongly repelled.
U.S. Pat. No. 3,454,493 discloses macroreticular resins achieved by copolymerizing monoethylenically unsaturated monomers with polyvinylidene monomers in the presence of a precipitant by the phase-separating technique as defined heretofore. The anion exchange resins subsequently formed are characterized by having average pore diameters in the range of 10,000 to about 500,000 Angstroms. The Example set forth in this patent discloses a macroreticular resin having a range of pore diameters from 30,000 to 200,000 Angstroms and a mean pore diameter of 70,000 Angstroms. The macroreticular anion exchange resin is claimed to remove colloidal particulate matter from aqueous media.
European Patent Application 0,135,292 discloses crosslinked macroporous resins using water-soluble polymeric porogens (such as poly (vinyl methyl ether) which are soluble in polymerizing monomers under the conditions of polymerization and which are at least partially soluble in water under certain physical conditions to facilitate removal following polymerization. After separation of the resulting copolymer from the mixture, any remaining porogen is extracted with water as solvent. The polymeric porogens disclosed in the European Patent Application are described as being more effective porogens at lower levels than polystyrene porogens or other precipitants previously used in the art as porogens, yielding greater porosities. The resulting resins are disclosed as having a mean pore radius as high as 2,052 Angstoms.